Heaving to is, like you suggest, backwinding the jib and leaving the main pulled in tight. The jib pushes the bow down until the main catches the wind, then that pushes the bow up into the wind until it starts to luff and the jib catches the wind and pushes the bow down again. The exact trim of each sail depends a bit on the boat and the wind. Every boat and every breeze is different, and you have to do what works in your particular situation. The wheel or tiller is also tied off so that the boat balances nicely between the two headings, and you can go below to dry off or sleep, or whatever. The boat (theoretically) moves gently along, "fore-reaching" at perhaps a knot of speed, but not really going much of anywhere and not putting big strains on the sails or rigging, even in strong winds. Heading up is simply turning the boat into the wind, possibly so far as to make the sails luff. In a strong breeze, the wildly flapping sails can be damaged. The boat slows and stops, perhaps even being pushed backwards by the wind and waves that it is heading into. Getting pushed backwards by a big wave can damage the rudder, which was designed to deal with forces coming from forward, not aft. Hope this helps.